In recent years, lithium secondary batteries which have grown in use as power supplies for driving portable electronic devices, communication devices, and the like are configured as secondary batteries with a high potential and a high discharge capacity by using a carbonaceous material capable of inserting and extracting lithium for a negative electrode plate and, composite oxide such as LiCoO2 or the like of transition metal and lithium for an active material of a positive electrode plate. However, with recent increase in function of electronic devices and communication devices, increased demands have been placed on the safety of battery while the capacity of lithium secondary batteries is further increased.
When an external physical impact is added to a lithium secondary battery by error for some reason, or when a lithium secondary battery is charged with an excessively large current, damage to a separator in the battery and the like might be caused to bring a positive electrode plate and a negative electrode plate in contact with each other, and thus an internal short-circuit might occur. When such an internal short-circuit occurs, a current flows intensively at short-circuited part and, as a result, heat is generated. If a large amount of heat is generated due to the internal short-circuit, materials forming the positive electrode and the negative electrode might be decomposed, an electrolyte might be caused to boil, and gas might be generated due to the decomposition of materials forming the positive electrode and the negative electrode. Thus, an internal short-circuit is considered as a factor for rapid heat generation in a battery.
To cope with this, a lithium secondary battery (see Patent Document 1) in which a porous protective film is formed on one of respective surfaces of a negative electrode active material layer and a positive electrode active material layer to ensure the safety of the battery has been proposed. Also, a lithium secondary battery (see Patent Document 2) in which a porous protective film formed of an inorganic filler (an inorganic insulation material) and a binder is attached to a surface of a negative electrode plate to suppress the occurrence of an internal short-circuit and which is provided with excellent heat resistance and thus exhibits improved safety has been proposed.
As a method for increasing a battery capacity, a method in which for each of a positive electrode plate and a negative electrode plate, a mixture paste which is a coating material obtained by mixing constituent materials is applied to a current collector core material and is dried and then a layer of the mixture paste is compressed by roll pressing to a predetermined thickness has been employed. However, as in this method, when a filling density of an active material in each of the electrode plates is increased, a porosity in each of the electrode plates is reduced and it becomes difficult to impregnate of an electrolyte in each of the electrode plates. Accordingly, impregnation of the electrolyte into an electrode plate group is dramatically reduced, thus resulting in undesirable nonuniform distribution of the electrolyte in the electrode plate group.
As an approach to solve the above-described problem, it has been proposed to form electrolyte guiding grooves each of which is formed so as to be concaved in an impregnation direction of electrolyte in an entire surface of a negative electrode active material layer (see Patent Document 3). Also, a nonaqueous electrolyte battery in which groove-like concave portions are provided in a surface of an active material layer in a positive electrode plate or a negative electrode plate to facilitate impregnation of an electrolyte into the electrode plate through the concave portions has been proposed (see Patent Document 4).    Patent Reference 1: Japanese Published Application No. H07-220759    Patent Reference 2: International publication WO 2005/029614    Patent Reference 3: Japanese Published Application No. H09-298057    Patent Reference 4: Japanese Published Application No. H11-154508